1. Field of the Invention
The present invention relates to a semiconductor device having a function to control, by a transistor, current to be supplied to a load. The present invention particularly relates to a display device including: a scan line driving circuit; a signal line driving circuit; and at least one of a pixel formed by a current-driving type display element of which luminance is changed by a signal, a pixel formed by a voltage-driving type display element of which luminance is changed by voltage, and a pixel formed by a display element of which transmittance is changed by voltage, such as a liquid crystal.
2. Description of the Related Art
In recent years, a so-called self-light-emitting display device in which a pixel is formed using a display element such as a light-emitting diode (LED) has attracted attention. As a display element used for such a self-light-emitting display device, for example, an organic light-emitting diode (also referred to as an OLED, an organic EL element, or an electroluminescent element) has attracted attention, and has been used for an EL display and the like. Since a display element such as an OLED is of self-light-emitting type, such a display device has advantages over a liquid crystal display in point of high visibility, no backlight required, and high response speed. It is to be noted that the luminance of a display element is controlled by the value of a current flowing through the display element.
A pixel matrix circuit of a general display device and its operation will be hereinafter described.
A pixel matrix circuit has a signal line driving circuit 7001, a scan line driving circuit 7002, and a pixel portion 7003. The pixel portion 7003 is provided with a plurality of pixels 7004 (FIG. 61). The plurality of pixels 7004 are arranged in a matrix form in accordance with scan lines (G1 to Gm) arranged in a row direction and signal lines (S1 to Sn) arranged in a column direction. The signal line driving circuit 7001 outputs video signals to the signal lines S1 to Sn, and the scan line driving circuit 7002 outputs to the scan lines G1 to Gm signals for selecting the pixels 7004 arranged in the row direction. Then, each of the video signals from the signal line driving circuit 7001 is written in the pixel corresponding to each column of the selected row. Each pixel stores the written signal.
In a similar manner, the signals are written in the pixels of every column in the rows selected sequentially. When signal writing is completed to all the pixels of the pixel portion 7003, a writing period to the pixels 7004 is completed. While the pixels are operated to emit light, the pixels 7004 store the written signals for a certain period. Therefore, each of the pixels 7004 maintains a state in accordance with the signal written therein. Then, by repeating the writing operation and light-emitting operation, a moving image is displayed.
The output of the video signal to the pixel is controlled by the signal line driving circuit 7001. The signal line driving circuit 7001 has, for example, a pulse output circuit 7011, a first latch circuit portion 7012, and a second latch circuit portion 7013. The pulse output circuit 7011 sequentially outputs sampling pulses to the first latch circuit portion 7012 in accordance with the timing of an inputted start pulse signal (S_SP) or the like. A video signal (video Data) is inputted to the first latch circuit portion 7012. The timing thereof is controlled in accordance with the sampling pulse outputted from the pulse output circuit 7011. Then, the video signal is held in each stage of the first latch circuit portion 7012. That is to say, a latch circuit of each stage of the first latch circuit portion 7012 operates based on the sampling pulse outputted from the pulse output circuit 7011.
After that, when the video signal input is completed to the last stage in the first latch circuit portion 7012, latch pulses (Latch Pulses) are inputted to the second latch circuit portion 7013, and the video signals held in the first latch circuit portion 7012 are simultaneously transferred to the second latch circuit portion 7013 and held in the second latch circuit portion 7013. Then, the video signals (for one row) are outputted simultaneously from the second latch circuit portion 7013 to the signal lines S1 to Sn. Then, while the signals are outputted from the second latch circuit portion 7013 to the signal lines, video signal data for the next row is inputted to the first latch circuit portion 7012. Then, after the input to the last stage, signals are transferred from the first latch circuit portion 7012 to the second latch circuit portion by latch pulses. By repeating this operation, the signals are inputted to all the pixels to display a moving image.
As a method for driving such a display device to express a gray scale, there are an analog gray scale method and a digital gray scale method. The analog gray scale method includes a method of controlling the light emission intensity of a display element in an analog manner and a method of controlling the light emission time of a display element in an analog manner. As the analog gray scale method, the method of controlling the light emission intensity of a display element in an analog manner is often used. However, the method of controlling the light emission intensity in an analog manner is easily affected by variation in characteristics of a thin film transistor (hereinafter also referred to as a TFT) between pixels, which causes variation also in luminance between pixels. On the other hand, in the digital gray scale method, a display element is turned on/off by controlling in a digital manner to express a gray scale. In the case of the digital gray scale method, the uniformity of luminance of each pixel is excellent. However, there are only two states, that is, a light emitting state and a non-light emitting state; therefore, only two gray scale levels can be expressed. Therefore, multiple-level gray scale display is attempted by using another method in combination. As a technique for multiple-level gray scale display, for example, there are an area gray scale method in which light emission area of a pixel is weighted (one pixel is divided into a plurality of regions and whether light emission or non light emission is controlled for every region) and selected to perform gray scale display and a time gray scale method in which light emission time is weighted (one frame is divided into a plurality of subframes and whether light emission or non light emission is controlled for every subframe) and selected to perform gray scale display. In the case of the digital gray scale method, the time gray scale method, which is also suitable to obtain higher definition, is often used (see, for example, Reference 1: Japanese Patent No. 2784615).
Here, improvement in definition can be achieved by using the time gray scale method in the digital gray scale method. However, as improvement in definition proceeds, the number of pixels is increased. Therefore, the number of pixels to which signal writing is conducted is also increased. Moreover, for higher-level gray scale display, the number of subframes needs to be increased. Accordingly, the number of times to write signals in pixels increases.
Moreover, in the aforementioned display device, since the pulse output circuit inputs sampling pulses for one row to the first latch circuit portion in all the rows, the pulse output circuit operates to transfer the signals for one row from the first to last columns. Thus, the increase in power consumption becomes a problem with the increase in the number of pixels.